1. Field of the Invention
This invention relates to an image recording apparatus in which a recording sheet is held on an interior surface of a cylinder and scanned by a plurality of light beams modulated according to image information.
2. Description of the Related Art
To record images on a recording sheet, there are used several types of recording apparatuses: a type in which a flat recording sheet being fed in an auxiliary scanning direction is scanned by a laser beam in a main scanning direction; another type in which a recording sheet held on a cylindrical surface is scanned by a laser beam; and a still other type of image recording apparatus in which a recording sheet is held on an interior surface of a cylinder and scanned by a laser beam (hereinafter referred to as a "cylindrical interior surface scanning type"). Among these apparatuses, the cylindrical interior surface scanning type apparatus has been increasingly used because of its superiority in recording precision, speed, economy, and so on. In the recording apparatus of this type, the recording sheet is rarely peeled off from a holding surface during the recording, since the recording sheet is held inside of the cylinder.
In the cylindrical interior surface scanning type image recording apparatus, the laser beam is introduced along the central axis of the cylinder to an optical scanning device disposed on and rotated about the central axis of the cylinder. The optical scanning device comprises a mirror surface inclined substantially at 45.degree. with respect to the incident direction of the laser beam to deflect the laser beam 90.degree. toward the recording sheet held on the interior surface of the cylinder, thereby to scan it.
There is an attempt, in the image recording apparatus of this type, of simultaneously scanning the recording sheet with a plurality of laser beams to increase the recording speed. However, when the plurality of laser beams are simply introduced to the optical scanning device, each of the laser beams deflected by the mirror surface produces a non-linear scanned locus on the recording sheet, and it is impossible to properly record images on the recording sheet.
Specifically, as shown in FIG. 15A, when the mirror surface 4 poses an illustrated attitude, with a shorter diameter of an elliptic mirror surface lying along the Y axis, three laser beams #1, #2 and #3 parallel to the Z-axis and incident upon the mirror surface are reflected along the X-Y plane, producing respective scanned loci lying one upon another on the recording sheet S. When the mirror surface 4 is rotated by 90.degree. from the attitude shown in FIG. 15A to that of FIG. 15B, the laser beams #1, #2 and #3 are reflected along the Y-Z plane producing respective scanned loci separated from each other on the recording sheet S. When the mirror surface 4 is further rotated by 90.degree. from the attitude shown in FIG. 15B to that of FIG. 15C, the laser beams #1, #2 and #3 are again reflected along the X-Y plane, producing respective scanned loci lying one upon another on the recording sheet S. As shown in FIG. 16, the laser beam #2 incident upon the mirror surface at its center of rotation has a scanned locus of a straight line on the recording sheet S, while the rest of laser beams, #1 and #3, incident upon the mirror surface at points other than its center of rotation have scanned loci varying depending on the attitude of the mirror surface 4, resulting in curved lines.
To avoid the above-described difficulty encountered when a plurality of laser beams are used, several proposals have been made.
According to the first of them, a hologram for adjusting deflection angles depending on incident positions of the laser beams is interposed between laser beam sources and an optical scanning device, the hologram rotating together with the rotation of the optical scanning device, thereby adjusting the incident positions of the laser beams upon the optical scanning device to make the scanned loci straight lines on the recording sheet (Japanese Patent Publication No. 4-73829).
According to the second of the proposals, P- and S-polarized laser beams are generated. One laser beam is led to a recording sheet via a polarizing beam splitter which is rotating. The other laser beam separated by the polarizing beam splitter is led to the recording sheet via a reflecting surface which is rotating (Japanese Laid-Open Patent Publication No. 5-27188).
According to the third of the proposals, P- and S-polarized laser beams are respectively modulated according to image information. One laser beam is then led to a recording sheet via a hologon (a holographic diffraction grating) which is rotating, and the other laser beam is led to the recording sheet via the hologon and a mirror whose angle is controlled by a piezoelectric device in accordance with the rotational angle of the hologon, thereby to maintain proper interval between the two laser beams on the recording sheet (U.S. Pat. No. 5,097,351).
According to the fourth of the proposals, a light scanner is provided which leads laser beams onto a recording sheet after reflecting the laser beams three times, thereby to cancel twists between the laser beams (Japanese Laid-Open Patent Publication Nos. 5-27190, 5-289018 and 5-308488).
According to the fifth of the proposals, a rotating light optical scanning device having a transparent layer on a reflecting surface is provided, and a plurality of laser beams having different wave lengths are respectively led to the rotating light optical scanning device. The laser beams are separated from each other owing to the difference in refractive indexes between them and led to a recording sheet (Japanese Laid-Open Patent Publication Nos. 4-348311 and 6-95016).
However, some of these prior art apparatuses suffer difficulty in that complexity of the apparatus is increased and accuracy of recording images is limited due to multiplicity of components included therein which are rotated together with the optical scanning device. In the apparatuses disclosed by Japanese Laid-Open Patent Publication Nos. 4-348311 and 6-95016, for example, which are relatively simple in structure, however, precise control of the wave lengths is very difficult, and hence the intervals between the scanned loci of the laser beams on the recording sheet.
In the apparatus disclosed by U.S. Pat. No. 5,097,351, though it is possible to control the intervals between scanned loci of the laser beams, there is no specific description on how to control the positions of scanned loci on the recording sheet. Therefore, accurate recording of images is thought to be difficult.
Further, the apparatuses disclosed in Japanese Laid-Open Patent Publication No. 5-27188 and U.S. Pat. No. 5,097,351 suffer shortcomings in that a remarkable increase in recording speed cannot be hoped for since only two laser beams are controlled in these apparatuses.